1. Field of the Invention
The present invention relates generally to wideband dual-polarized antennas having horizontal and vertical elements which are suitable for transmitting and receiving short electromagnetic pulse (EMP) signals, and more particularly, to a dual-polarized antenna having a ground plane that separates dipole elements from one another and tilts the radiating elements.
2. Description of the Prior Art
It is well known that an antenna acts as a transformer between free space and a transmitter or receiver. It is also known that the efficiency of transformation depends in part on how well an antenna is matched to the feed line which connects the antenna to a transmitter or receiver. Matching the impedance of an antenna to a feed line typically involves an understanding of signal characteristics and the particular antenna employed, both of which are dictated by the specific application. Considerations of bandwidth, dispersion, polarization, and physical size may or may not be important depending on the intended application.
Common types of antenna include the biconical dipole, the monopole discone, spirals and helicals, the single and crossed Yagi-Uda, the TEM horn, and the familiar log-periodic antenna. The characteristics of the electromagnetic signal which is broadcasted or detected determines which of these antennas is suitable for a given application. For example, an antenna for use in a pulse radar unit must be able to transmit and receive short electromagnetic pulse (EMP) signals. Since EMP signals are composed of all frequencies, a radar antenna must have a wide frequency response. Antennas having a wide frequency response are commonly referred to as being wideband or broadband antennas. For purposes of consistency, we regard all antennas having a bandwidth greater than about 90% as being broadband, where ##EQU1## .DELTA.f being the frequency difference between half-power points, and f.sub.0 the center frequency of the antenna. By this criteria the biconical dipole, monopole discone, spiral, helical, TEM horn, and log-periodic type antennas are all broadband.
In addition to being broadband, it is also desirable for a radar antenna to be fully polarimetric. A fully polarimetric antenna is capable of transmitting and receiving vertically and horizontally (i.e. orthogonally) polarized electromagnetic waves simultaneously. Thus, fully polarimetric antennas are sometimes referred to as being dual-polarized. Polarization is an important consideration in radar since it is difficult to predict a priori whether an unseen target presents a primarily vertical or horizontal profile. While single feed Yagi-Uda and log-periodic antennas are linearly polarized, crossed Yagi-Uda and log-periodic antennas have twin feeds and are fully polarimetric. However, neither the single feed nor the crossed Yagi-Uda antennas is broadband. Helical, spiral, biconical, and discone antennas have heretofore been either linearly or circularly polarized, making them less than ideal choices for a radar antenna.
It is also desirable for a radar antenna, and indeed any EMP antenna, to be non-dispersive. Dispersion, also known as group delay, concerns the speed of electromagnetic radiation and its variation with frequency. A non-dispersive antenna transmits all frequencies in such a way that they travel together. Thus, a short pulse fed into a non-dispersive antenna will retain its step-like characteristic when received by another non-dispersive antenna. Dispersive antennas, on the other hand, can decrease the rise time of a signal thereby "softening" a step pulse input into a rounded chirp transmission. Sharp pulses are preferred in radar and communications since they are more easily discerned and analyzed. Only the TEM horn, biconical dipole and monopole discone antennas are non-dispersive, and none of these is fully polarimetric.